Photolithographic structure generation process

ABSTRACT

In a process for photolithographic generation of structures in the sub-200 nm range, a layer of amorphous hydrogen-containing carbon (a-C:H) with an optical energy gap of &lt;1 eV or a layer of sputtered amorphous carbon (a-C) is applied as the bottom resist to a substrate (layer thickness ≦500 nm); the bottom resist is provided with a layer of an electron beam-sensitive silicon-containing or silylatable photoresist as the top resist (layer thickness ≦50 nm); the top resist is structured by means of scanning tunneling microscopy (STM) or scanning force microscopy (SFM) with electrons of an energy of ≦80 eV; and then the structure is transferred to the bottom resist by etching with an anisotropic oxygen plasma and next is transferred to the substrate by plasma etching.

BACKGROUND OF THE INVENTION

The invention concerns a process for photolithographic generation ofstructures in the range below 200 nm.

In semiconductor technology and in microelectronics, the dimensions ofstructures are becoming smaller and smaller. In memory production today,structures with a width of less than 400 nm are produced using opticallithography in combination with the masking technique. A limit can beexpected at approx. 150 nm in optical lithography because of diffractioneffects. However, structures with even smaller dimensions are requiredfor new applications such as single-electron transistors or molecularelectronic components. In the case of very high-frequency circuits thisis also true in conventional electronics.

One method offering possibilities for generating such small structuresis X-ray lithography, which makes it possible to image dimensions ofless than 100 nm--because of the shorter wavelength. However, this leadsto problems with the required masks and in positioning. This is not thecase with electron and ion beam lithography. Since these are directwriting methods, they do not require any masks. In electron and ion beamlithography, structures as small as 10 nm can be generated withhigh-energy particles. However, this requires expensive vacuum systemsand beam guidance systems. In addition, problems can occur withsensitive components due to radiation damage in the substrate, becausethe high-energy particles can penetrate through the resist layersrequired for etching processes.

A novel possibility for high-resolution structuring is presented byscanning near-field techniques, in particular scanning tunnelingmicroscopy (STM) and scanning force microscopy (SFM). With regard towriting speed, these techniques are in principle slower than electronbeam writing methods but it is possible to work in parallel with aseries of near-field probes. In addition, the writing speed and thus thetime should not be crucial for initial developments because the methodyields the great advantage that expensive vacuum systems can beeliminated.

With scanning near-field techniques, a fine pointed probe is moved overthe surface of the specimen at a constant distance, and topographicdifferences can be compensated in this way. Interactions between thespecimen surface and the tip of the probe serve to regulate thedistance. When these techniques, which are usually employed only forscanning surface topographies, are to be used as structuring methods,then a current flow is generated, using an externally applied voltage,from the tip of the probe into the specimen or vice versa, depending onthe polarity of the voltage, causing a chemical or physical change inthe specimen surface. Since the distance between the tip of the probeand the surface of the specimen is extremely small, collisions betweenthe electrons emitted and the molecules of air can be disregarded, andtherefore electron exposure of the specimen surface is possible withscanning near-field techniques not only in high vacuum but also atnormal pressure. This is an important advantage in comparison with theactual electron beam writing, where high vacuum is required--to preventcollisions between electrons and residual gas molecules in theacceleration zone (1-50 kV)--which entails a considerable expenditure.

With scanning near-field techniques, the electrons emitted cannot beaccelerated to a high kinetic energy. In other words, if the voltageapplied to the probe is too high (approx. >80 V), uncontrolled damagemay occur to the specimen and probe. Thus only electrons with an energyof up to approx. 80 eV may be used for electron bombardment withscanning near-field probes. This energy level is sufficient to initiatechemical changes in conventional electron beam-sensitive resistmaterials, but it is not great enough for the electrons to penetratethrough dielectric resist layers more than a few 10 nm thick. Theadvantage that radiation damage in the substrate is ruled out has thusso far been outweighed by the disadvantage that only extremely thinresist layers can be used.

In microelectronics, however, plasma processing methods such as reactiveion etching (RIE) are conventional and require structurable,etching-resistant masks in the form of resists with a layer thicknessof >100 nm, depending on the etching depth of the structures to beproduced.

Therefore, when using scanning near-field techniques, it is necessary toeither refrain from substrate etching by plasma etching processes, inwhich case, however, the aspect ratio of the structures produced(height/width) is limited to values of <1--or the etching processes mustbe performed in such a way that the etching depth does not exceed thethickness of the resist (see P. Avouris (ed.) Atomic and Nanometer-ScaleModification of Materials. Fundamentals and Applications, KluwerAcademic Publishers, 1993, pages 139-148). Another possibility consistsof using metal halides, in particular calcium fluoride (CaF₂), whichhave the etching stability required for substrate etching processes evenin a thin layer (see Journal of Vacuum Science & Technology B, vol. 5(1987), pages 430-433). However, the lithographic properties of suchinorganic materials are very poor, in particular the dose required forstructuring is very high, amounting to 1 C/cm² for a 20 nm thick CaF₂resist, for example. This in turn limits the writing speed and thereforethe throughput.

SUMMARY OF THE INVENTION

The object of this invention is to provide a process that will make itpossible to produce sub-200 nm structures with a high aspect ratio bymeans of low-energy electrons at normal pressure while at the same timepermitting structuring of thick resist layers (>100 nm).

This is accomplished according to this invention by applying a layer ofamorphous hydrogenous carbon with an optical energy gap of <1 eV or alayer of sputtered amorphous carbon to a substrate as a bottom resist,where the layer thickness is ≦500 nm, providing the bottom resist with alayer of an electron beam-sensitive, silicon-containing or silylatablephotoresist with a layer thickness of ≦50 nm as a top resist,structuring the top resist by scanning tunneling microscopy or scanningforce microscopy with electrons of an energy of ≦80 eV, and transferringthe structure first to the bottom resist by etching with an anisotropicoxygen plasma and then to the substrate by plasma etching.

DETAILED DESCRIPTION OF THE INVENTION

The process according to this invention contains a series of importantmeasures. Thus, a two-layer resist system with a top resist (top layer)and a bottom resist (bottom layer) is used. The top resist is a thinelectron beam-sensitive photoresist (layer thickness ≦50 nm). Thisphotoresist, which may be a positive or negative resist, either containssilicon or can be silylated. A silylatable resist offers the advantagethat the layer thickness can be increased by a chemical aftertreatmentand the etching stability in the oxygen plasma can be increased. Thebottom resist is a relatively thick layer (≦500 nm) of amorphoushydrogenous carbon (a-C:H) and having an optical energy gap of <1 eV, orsputtered amorphous carbon (a-C). Such a resist material has thefollowing properties:

it has a sufficient electric conductivity,

it can be applied as a thin, homogeneous, closed film,

it does not contain any ionic impurities,

it is not attacked by the solvent for the top resist,

it can easily be etched in an oxygen plasma without leaving a residue,

it has a high stability in etching processes with halogen-containingplasmas.

Layers of a-C:H with the properties described above can be created bydeposition from a hydrocarbon plasma, specifically according to aso-called PECVD process (plasma enhanced chemical vapor deposition). Theprocess parameters, i.e., the type of gas, the gas pressure and theself-bias voltage--determined by the power and the reactor geometry--areselected so that this material has a sufficiently small optical energygap (<1 eV) or a sufficient electric conductivity (≧10⁻⁵ Ω⁻¹ ·cm⁻¹). Asa result, low-energy electrons can be removed through the bottom resistlayer.

Suitable a-C layers can be obtained by sputtering from a graphite targetin a conventional sputtering system, where sputtering parameters thatyield smooth adhesive layers are maintained. The substrate on which thematerial is deposited must optionally be pretreated by a suitablecleaning process (argon or oxygen plasma).

In the process according to this invention, the top resist is irradiatedand structured with low-energy electrons (voltage ≦80 V) by means of ascanning tunneling microscope or a scanning force microscope. Theradiation dose advantageously used is 1-50 mC/cm², preferably 10-30mC/cm². The structure produced is then transferred with the help of ananisotropic oxygen plasma (O₂ -RIE) to the bottom resist, i.e., to thea-C:H layer or the a-C layer, and is then transferred to the substrateby plasma etching. Transfer of the structure to the substrate isgenerally accomplished by means of a halogen plasma such astetrafluromethane (CF₄); other suitable halogen plasmas include chlorine(Cl₂), boron trichloride (BCl₃) and sulfur hexafluoride (SF₆). An oxygenplasma is used for transfer of the structure to substrates such aspolyimide.

The measures described above yield the effect that great etching depthscan be achieved in the substrate with a very thin photoresist. Theetching depth in the substrate depends on the etching selectivity of thesubstrate in comparison with the bottom resist layer (of a-C:H or a-C).Since a-C:H layers may be under stress, they are deposited in principleonly on materials where there is good adhesion. Deposition on the mostcommon substrate materials in semiconductor technology, such asmonocrystalline silicon, polycrystalline silicon, SiO₂, quartz, Si₃ N₄,SiC, aluminum and polyimide, is possible without problems; with galliumarsenide, the layer thickness that can be deposited is generally limitedto values of less than 200 nm. For the etching selectivity (substrate:a-C:H) in a plasma excited by radiofrequency, the following values areobtained, for example: polycrystalline silicon 6:1 (Cl₂ ; 800 W/3.7μbar), SiO₂ 20:1 (CF₄ ; 700 W/5 μbar), aluminum 5:1 (SiCl₄ +Cl₂ +N₂ ;360 W/0.16 mbar), polyimide 2:1 (O₂ ; 900 W/3 μbar). On copper andtungsten, a-C:H cannot be deposited directly, so in these cases anintermediate layer of aluminum is used.

Layers of a-C have a lower stress and can therefore be deposited on allconventional substrate materials, including copper and tungsten. Theselayers have an etching stability comparable to that of a-C:H. However,layers of a-C are not permeable to visible light, so it may be difficultto locate adjusting marks, and optical layer thickness measurements areimpossible under some circumstances.

Materials that need only a low dose for exposure are preferred as theelectron beam-sensitive top resists. This is the case in particular withresists that work according to the principle of chemical amplification(chemically amplified resists). Writing time can be minimized with suchresists, which are also used in electron beam lithography (dose requiredat 30 keV: <10 μC/cm²).

It is advantageous to use top resists that can be silylated, which isperformed after structuring. Such a resist has functional groups thatare capable of reacting with agents containing silicon. Such groupsinclude in particular anhydride groups, which can be reacted withaminopropylsiloxanes, for example, and hydroxyl groups that can bereacted with silazanes. For chemical aftertreatment of the resists,solutions of functional silicon compounds of the aforementioned type areused, such as those known for the corresponding resist materials (see,for example, Polymer Engineering and Science, vol. 32 (1992) pages1558-1564, and Journal of Vacuum Science & Technology B, vol. 10 (1992)pages 2610-2614). Moreover, spaces (positive images) or lines (negativeimages) may also be produced, depending on how the resist process iscarried out.

The invention is explained in greater detail below on the basis of thefollowing examples.

EXAMPLE 1

A silicon wafer is coated with amorphous hydrogen-containing carbon(layer thickness: 450 nm)--with an optical energy gap of 0.85 eV--in amethane-filled PECVD system (parallel plate reactor, 3-inch cathode witha 13.56 MHz transmitter, anode six times larger); deposition parameters:-900 V self-bias voltage, 0.1 mbar methane pressure, 7 min depositiontime. Then a 45 nm thick layer of an electron beam-sensitive resist isapplied to this layer by spin coating and dried at 120° C. for 60 s on ahot plate; the resist is based on a basic polymer with anhydride groupsand tert-butyl ester groups (see U.S. patent application Ser. No.08/386,136 . The top layer is then "exposed" by means of a scanningtunneling microscope (STM device) by the fact that the tip of the probeto which -50 V is applied scans the specimen at the rate of 1 μm/s andthus "writes" lines (current: 10 pA). Then the wafer is heated at 120°C. for 120 s on a hot plate and then developed for 60 s in an aqueousdeveloper solution of tetramethylammonium hydroxide (1.19%) and1-phenylethylamine (0.25%), where 150 nm wide spaces are formedaccording to the scan pattern (positive image); the dark field erosionin developing amounts to 5 nm. Next, the wafer is treated for 60 s witha silylation solution consisting of a 4% solution ofdiaminopropyldimethylsiloxane oligomer in a mixture of 2-propanol andethanol (ratio 2:1). The layer thickness of the resist increases by 45nm to a total of 85 nm, and the space width decreases from 150 nm to 80nm. The structures produced in this way are transferred to the amorphoushydrogen-containing carbon layer by means of a plasma etching system(model MIE 720, MRC) at an RF power of 900 W (45 V self-bias voltage)and an oxygen flow of 30 sccm (3 μbar). The etching time is 84 s(including 50% overetching), where the thickness of the photoresistlayer decreases to 15 nm and the space width of the structures increasesto 100 nm. The aspect ratio of the structures in the carbon layer isthus 4.5. The resist structure is then transferred to the silicon waferby means of a CF₄ plasma, where the maximum possible etching depth isdetermined by the resist layer thickness and the etching selectivity; inthe present case this is 6×450 nm. In practice, however, the etchingdepth amounts to less than 50% of the maximum value.

The structure widths of approx. 100 nm that can be achieved can befurther greatly reduced by optimizing the writing parameters. Because ofthe lack of a proximity effect, a greater resolution can be expected inSTM and SFM lithography than with high-energy electrons (50 keV).

EXAMPLE 2

A silicon wafer is coated with amorphous hydrogen-containing carbon(layer thickness 200 nm) with an optical energy gap of 0.85 eV in amethane-filled PECVD system (parallel plate reactor, 3-inch cathode witha 13.56 MHz transmitter, anode six times larger); deposition parameters:-900 V self-bias voltage, 0.1 mbar methane pressure; 3 min depositiontime. Then a 45 nm thick layer of a commercial electron beam-sensitiveresist based on novolak (AZ 5214E, diluted 1:6 with methoxypropylacetate) is applied to this layer by spin coating and dried at 110° C.for 90 s on a hot plate. The top layer is then "exposed" by means of ascanning force microscope (SFM device) by scanning the specimen with thetip of the probe, which receives -35 V, at the rate of 3 μm/s and thus"writing" lines (current: <100 pA). Then the wafer is heated at 130° C.for 90 s on a hot plate, exposed over the entire area with near UV light(20 mJ/cm²) and then developed for 20 s in a commercial aqueous alkalinedeveloper solution (AZ 400K, diluted 1:4 with water). Next, the wafer istreated for 60 s with a silylation solution consisting of a 12% solutionof bis(dimethylamino)dimethylsilane in a mixture of methoxypropylacetate and n-decane (ratio 1:1). The layer thickness of the resistincreases by 15 nm to a total of 60 nm. The structures produced in thisway are transferred to the amorphous hydrogen-containing carbon layer bymeans of a plasma etching system (model MIE 720, MRC) at an RF power of900 W (45 V self-bias voltage) and an oxygen flow of 30 sccm (3 μbar).The etching time here is 38 s (including 50% overetching), where thethickness of the photoresist layer decreases to 20 nm and lines with astructure width of 100 nm are formed (negative image). The aspect ratioof the structures in the carbon layer is thus 2. The resist structure isthen transferred to the silicon wafer by means of a CF₄ plasma, wherethe maximum possible etching depth is determined by the resist layerthickness and the etching selectivity; in the present case this is 6×200nm. In practice, however, the etching depth is less than 50% of themaximum value.

EXAMPLE 3 (COMPARATIVE EXPERIMENT)

A silicon wafer is coated with amorphous hydrogen-containing carbon(layer thickness 250 nm) with an optical energy gap of 1.1 eV in amethane-filled PECVD system (parallel plate reactor, 3-inch cathode witha 13.56 MHz transmitter, anode six times larger); deposition parameters:-650 V self-bias voltage, 0.15 mbar methane pressure, 4.5 min depositiontime. A 45 nm thick layer of an electron beam-sensitive resist accordingto Example 1 is applied to this layer by spin coating and dried at 120°C. for 60 s on a hot plate. In an attempt to "expose" the top layer bymeans of a scanning tunneling microscope (STM device) according toExample 1, charging effects that limit the maximum current are observed.Therefore, the structures cannot be developed completely to the a-C:Hlayer.

EXAMPLE 4

In a sputtering system (model Z 550, Leybold) with a rotatablesputtering target (made of carbon), the substrate disk is loaded with asilicon wafer coated with aluminum, then the chamber is evacuated with aturbo pump to a pressure of 9×10⁻⁷ mbar. To clean the substrate (coatedwafer) a pressure of 5×10⁻³ mbar is established at an argon flow of 50sccm and an oxygen flow of 5 sccm, and then sputtering is performed at ahigh-frequency power of 300 W for 3 minutes with both process gases.Then the oxygen flow is stopped and sputtering is performed for 3minutes more with pure argon. For the coating process, a d.c. voltageplasma is ignited on the carbon target (argon flow 75 sccm) andsputtering is performed blind first (to clean the target) for 3 minutesat 500 W (pressure 7.1×10⁻³ mbar). Then the sputtering target is movedover the substrate and sputtering is continued for 900 s more, yieldinga very hard, adhesive amorphous carbon layer approx. 250 nm thick with asufficient electric conductivity. A 45 nm thick layer of an electronbeam-sensitive resist according to Example 1 is applied to this layer byspin coating and dried at 120° C. for 60 s on a hot plate. Then the toplayer is "exposed" by means of a scanning tunneling microscope (STMdevice) by scanning the specimen with the tip of the probe, whichreceives -50 V, at the rate of 1 μm/s and thus "writing" lines (current10 pA). Then the substrate is heated at 120° C. for 120 s on a hot plateand next developed for 60 s in an aqueous developer solution oftetramethylammonium hydroxide (1.19%) and 1-phenylethylamine (0.25%),yielding 150 nm wide spaces corresponding to the scan pattern (positiveimage); the dark field erosion in developing amounts to 5 nm. Next thesubstrate is treated for 60 s with a silylation solution consisting of a4% solution of diaminopropyldimethylsiloxane oligomer in 2-propanol. Thelayer thickness of the resist increases by 40 nm to a total of 80 nm,and the space width decreases from 150 nm to 90 nm. The structuresproduced in this way are transferred to the sputtered amorphous carbonlayer by means of a plasma etching system (model MIE 720, MRC) at an RFpower of 900 W (45 V self-bias voltage) and an oxygen flow of 30 sccm (3μbar). The etching time is 36 s (including 50% overetching), where thethickness of the photoresist layer decreases to 30 nm and the spacewidth of the structures increases to 100 nm. The aspect ratio of thestructures in the carbon layer is thus 2.5. The resist structure is thentransferred to the aluminum layer by means of a CF₄ plasma, where themaximum possible etching depth is determined by the resist layerthickness and the etching selectivity; in the present case this is 4×250nm. In practice, however, the etching depth is less than 50% of themaximum value.

EXAMPLE 5

A silicon wafer is coated with amorphous hydrogen-containing carbon(layer thickness 350 nm) with an optical energy gap of 0.85 eV in amethane-filled PECVD system (parallel plate reactor, 3-inch cathode witha 13.56 MHz transmitter, anode six times larger); deposition parameters:-900 V self-bias voltage, 0.1 mbar methane pressure, 7 min depositiontime. Then a 45 nm thick layer of an electron beam-sensitive resistaccording to Example 1 is applied to this layer by spin coating anddried at 120° C. for 60 s on a hot plate. The top layer is then"exposed" by means of a scanning tunneling microscope (STM device) byscanning the specimen with the tip of the probe, which receives -55 V,at the rate of 1 μm/s and thus "writing" lines (current 20 pA). Then thewafer is heated at 120° C. for 120 s on a hot plate and then treated for60 s with a silylation solution consisting of a 1% solution ofdiaminopropyldimethylsiloxane oligomer in a mixture of 2-propanol andwater (ratio 15.5: 1). The layer thickness of the resist increases by 40nm to a total of 80 nm in the "exposed" areas. The structures producedin this way are transferred to the amorphous hydrogen-containing carbonlayer by means of a plasma etching system (model MIE 720, MRC) at an RFpower of 900 W (45 V self-bias voltage) and an oxygen flow of 30 sccm (3μbar). The etching time is 74 s (including 50% overetching), where thethickness of the photoresist layer decreases to 45 nm and structureswith a line width of 80 nm are formed (negative image). The aspect ratioof the structures in the carbon layer thus amounts to 4.3. The resiststructure is then transferred to the silicon wafer by means of a CF₄plasma, where the maximum possible etching depth is determined by theresist layer thickness and the etching selectivity; in the present casethis is 6×350 nm. In practice, however, the etching depth amounts toless than 50% of the maximum value.

What is claimed is:
 1. A process for photolithographic generation ofstructures in the sub-200 nm range, comprising the steps of: applying alayer of amorphous hydrogen-containing carbon (a-C:H) having an opticalenergy gap of <1 eV to a substrate as a bottom resist having a layerthickness of ≦500 nm; applying to the bottom resist a layer of anelectron beam-sensitive silicon-containing or silylatable photoresisthaving a layer thickness of ≦50 nm as a top resist; exposing the topresist by scanning tunneling microscopy or scanning force microscopywith electrons of an energy of ≦80 eV to form a pattern afterdevelopment; transferring the pattern to the bottom resist by etchingwith an anisotropic oxygen plasma and then to the substrate by plasmaetching.
 2. The process according to claim 1, wherein the exposing isperformed with a radiation dose in a range of 1-50 mC/cm².
 3. Theprocess according to claim 2, wherein the pattern is transferred to thesubstrate by etching with a halogen plasma.
 4. The process according toclaim 3, wherein the substrate is a semiconductor material.
 5. Theprocess according to claim 3, wherein the top resist is treated with asilicon-containing agent after developing.
 6. The process according toclaim 2, wherein the substrate is a semiconductor material.
 7. Theprocess according to claim 6, wherein the top resist is treated with asilicon-containing agent after developing.
 8. The process according toclaim 2, wherein the top resist is treated with a silicon-containingagent after developing.
 9. The process according to claim 1, wherein theexposing is performed with a radiation dose in a range of 10-30 mC/cm².10. The process according to claim 1, wherein the pattern is transferredto the substrate by etching with a halogen plasma.
 11. The processaccording to claim 10, wherein the substrate is a semiconductormaterial.
 12. The process according to claim 10, wherein the top resistis treated with a silicon-containing agent after developing.
 13. Theprocess according to claim 1, wherein the substrate is a semiconductormaterial.
 14. The process according to claim 12, wherein the top resistis treated with a silicon-containing agent after developing.
 15. Theprocess according to claim 1, wherein the top resist is treated with asilicon-containing agent after developing.